This invention relates generally to an improved internal voltage divider structure for a television cathode ray tube. In its present preferred implementation, the voltage divider supplies one or more voltages intermediate to the high voltage of the ultor anode, and the relatively lower voltages introduced through the base of the tube. This invention has applicability to tubes of many types in construction, but is believed to be most advantageously applicable in color television cathode ray tubes having electron guns with extended field focus lenses requiring a plurality of focus lens potentials. The intermediate voltages supplied by the voltage divider according to the invention may also supply potentials to electrodes within the cathode ray tube other than gun electrodes, such as a focus mask electrode.
Electron guns of the type referred to in the foregoing may comprise a series of discrete, electrically conductive discs or tubular elements contiguous to each other and aligned on a common axis. In multi-gun assemblies, each gun may comprise a series of electrically discrete electrodes, or, the electrodes of gun which have functions in common may be physically combined, or "unitized." The electrodes receive voltages of a predetermined potential to establish electrostatic fields therebetween for forming and shaping the beam, and, in the main focus lens, for focusing the beam crossover to provide small, symmetrical "spots" on the viewing screen. The electron guns having extended field lenses of a certain type may require one or more selected potentials in the range of approximately eight to fifteen kilovolts or higher; these voltages are designated herein as "intermediate" to the relatively high ultor anode voltage of twenty-five to thirty-two kilovolts, and the relatively lower voltages in the one to eight kilovolt range introduced through the base of the cathode ray tube. If the said intermediate potential is brought into the tube through the base as shown, for example, in U.S. Pat. No. 3,995,194, such a tube cannot readily be installed in a television chassis having no provision for supplying such potentials; for example, a television chassis designed for a tube incorporating a gun with a bipotential lens requiring only relatively low and relatively high operating voltages as defined in the foregoing.
As noted, the aforesaid intermediate potential is commonly introduced into the envelope of the cathode ray tube through the base of the tube along with the relatively low voltages. The introduction of a potential of the magnitude cited through the base can engender serious problems. A major problem is a tendency toward destructive arcing in the base area due to the close spacing of high-voltage leads in the base, and, in the neck of the tube.
The structure and relationship of an electron gun and cathode ray tube, and the prior art means for supplying operating voltages to the combination, is shown by FIG. 1. The primary components of a typical color picture tube 10 comprise an evacuated envelope including a neck 12, a funnel 14 and a faceplate 16. On an inner surface of the faceplate 16 are deposited a multiplicity of cathodoluminescent phosphor target elements 18 comprising a pattern of groups of red-light-emitting, green-light-emitting, and blue-light-emitting dots or stripes. A perforated electrode 20 called a "shadow mask," is used in the tube for color selection. Base 22 provides entrance means for a plurality of electrically conductive lead-in pins 24.
An electron gun 26, illustrated schematically, is disposed within neck 12 substantially as shown. Gun 26 is commonly installed in axial alignment with a center line X--X of picture tube 10. Gun 26 emits one or more electron beams 28 to selectively activate target elements 18.
Power supply 30, also shown schematically, provides voltages for operation of the cathode ray tube and its electron gun. To supply the required potentials, a special voltage divider circuit is typically incorporated into the external power supply circuit. Power supply 30 may supply relatively low voltages in the one to eight kilovolt range through one or more leads represented schematically by 32, which enter the envelope of tube 10 through the plurality of lead-in pins 24 in base 22. Power supply 30 may also supply selected intermediate voltages to the focus electrodes of electron gun 26, voltages typically in the range of eight to fifteen kilovolts or higher; these voltages are shown as being supplied to the electrodes within the envelope of tube 12 by way of lead-in pins 24 through lead 33. The relatively high voltage for electron gun operation; that is, a voltage typically in the range of twenty-five to thirty-two kilovolts for excitation of the accelerating anodes, is indirectly supplied to gun 26 through lead 34, which is connected to anode button 36. Anode button 36 in turn introduces the high voltage through the glass envelope of funnel 14, making internal contact with a thin, electrically conductive coating 38 disposed on the internal surface of funnel 14, and part-way into neck 12. The anode electrode of gun 26 receives the relatively high anode voltage through a plurality of metallic gun centering springs 40 extending from gun 26 and in physical contact with inner conductive coating 38.
The introduction of the selected intermediate voltages described heretofore into the cathode ray tube envelope 10 through lead-in pins 24 has presented serious problems in prior art cathode ray tube technology. Introduction of such high voltages through the lead-in pins has typically required an elaborate socket, indicated schematically by 42 in combination with base 22 and associated lead-in pins 24. The close adjacency of lead-in pins 24, and the wide range of potentials thereon (from a few volts to many kilovolts) has made it necessary to devise tube socket-base combinations capable of shielding the lead-in pins one from the other. Isolation means have included insulative barriers or walls molded as part of the socket and base to extend prospective arc paths. Sockets have also comprised non-destructive arcing paths in their structure, and arc-quenching means embodied in the socket and base combination. It has also often been necessary to introduce potting compounds into the tube base to eliminate arc-prone air paths between leads. This complexity of the socket and base combination adds to manufacturing costs.
In U.S. Pat. No. 3,932,786 to Campbell, there is disclosed a voltage divider in conjunction with a bipotential electron gun for a television cathode ray tube. Comprising a single short resistive element, the voltage divider is shown as being mounted on one of the two glass support rods of the gun. The single resistive element of the voltage divider is electrically connected between the first and second accelerating and focus electrodes of the bipotential main focus lens. The resistive element has a series of six taps which are connected to six electrode plates successively spaced between first and second focus electrodes of the gun. The plates are electrically connected by means of electrical taps to the resistive element at points of successively greater resistance with respect to the end attached to the first focus electrode of the gun.
The composition and the operating requirements of the single resistive element according to Campbell are described as follows (quoting from column 2, lines 62-68, and column 3, lines 1 and 2): "Resistor 50 is a thin cermet film 49 deposited on a substrate 51 which is bonded to one of the glass support rods 28. In order to operate within the cathode ray tube, the resistor 50 must have a very small temperature coefficient of resistivity and must be able to withstand a high voltage (approximately 32,000 volts) that is applied to the second electrode 26."
The temperature coefficient of resistivity (resistance-temperature characteristic) is defined as the magnitude of change in resistance due to temperature, usually expressed in percent per degree Celsius or parts per million per degree centigrade (ppm.degree. C.). (Reference Data for Radio Engineers, Howard W. Sams & Co., Inc., 1970.) A large temperature coefficient implies a large change in resistance for a given change in temperature, while a small coefficient implies a small change. A small temperature coefficient is desirable in a resistor used as a voltage divider as the tapped-off voltage will, as a result, vary only minimally as a result of temperature change.
However, the environment within a cathode ray tube is one of wide temperature variance. When not operating, the temperature of the electron gun components in the neck of a cathode ray tube, for example, may be at the normal ambient temperature of about 22.degree. C. Following tube turn-on, the pattern of temperature distribution in the neck of the cathode ray tube will be fixed or will vary with time as a consequence of tube warm-up and operation, with the maximum temperature approaching 100.degree. C.
The effect of temperature differences on a single-resistive-element voltage divider comprised of cermet results in a substantial variation in resistance due to temperature change, and hence, a substantial temperature dependence in the potential at each tap-off point. The temperature coefficient of resistivity of a typical cermet material may be relatively large, e.g., .about.500 ppm.degree. C. Although lower values are attainable from certain special materials available in the art, they require great care in processing to avoid wide variations in performance.
The single resistive element comprising the Campbell voltage divider is shown as being quite short in relation to the axial length of the electron gun. A short resistive element offers the advantage of being relatively unaffected by temperature differences that may exist along the length of the gun. There are, however, marked, offsetting disadvantages inherent in a short resistor length. One disadvantage lies in the fact that the voltage per unit length is very large; e.g., twenty-eight kilovolts across the short resistive path of the Campbell voltage divider. Another disadvantage stems from the fact that there must be an appreciable current through the voltage divider so that stray currents do not alter the division ratio. A typical current would be 60 microamperes. Assuming twenty-eight kilovolts across the Campbell resistor, the resistor would have to dissipate about one and a half watts or more. Because of the short length of the resistor and its small mass, the resistive element could well become intolerably hot.
For the reasons given, the performance of a voltage divider having a single resistive element according to Campbell is considered to be not adequate for developing temperature-invariant voltages in a cathode ray tube, particularly in high voltage applications.
Generally with regard to the temperature dependence of electronic components, the problem of locating such components so as not to be adversely affected by a nearby heat-generating component, is recognized in microcircuit technology. The problem is described in chapter 13, pages 184-185 of Electronic Integrated Systems Design, by H.R. Camenzind. (New York: Van Nostrand Rhinehold, 1972). The problem is said to be especially significant in devices such as transistors which are expected to match. Camenzind suggests placing sensitive matching devices as far away from the heat source as possible, and locating the devices on an isothermal contour. An associated illustration in the Camenzind reference shows a "chip" having three spaced isothermal contours parallel to a long side of a rectangular heat-generating power device. Two transistors are shown positioned side-by-side on the isothermal contour farthest from the power device, and lying parallel with the contour.
U.S. Pat. No. 2,143,390 to Schroter is cited of interest only in that a potentiometer is disclosed which may be built inside a cathode ray tube. Similarly, U.S. Pat. No. 2,859,378 to Gundert et al is cited of interest only in that it discloses a voltage divider used to supply a multiplicity of potentials to the electrodes of an electron gun used in cathode ray tubes. The Gundert et al voltage divider is shown as being installed externally to the cathode ray tube envelope.